Weizmann Institute Astrophysicist Hot On The Tracks Of A New
'Strange' Star

REHOVOT, Israel, January 12, 1998 -- A new "strange" star is
suspected to be lurking in our galaxy -- and a Weizmann Institute
astrophysicist is hot on its tracks.

In research published in the January 12 issue of Physical Review
Letters, Prof. Vladimir Usov of the Institute's Condensed Matter
Physics Department outlines the last of three characteristics that
may enable astronomers to finally identify examples of strange
stars, whose existence was predicted nearly 15 years ago. The
first two characteristics were described by Usov in the June 1,
1997 issue of The Astrophysical Journal Letters.

"Only a handful of the heavenly bodies observed in our galaxy are
probably strange stars, but they would constitute a totally new
class of celestial objects with extraordinary properties," Usov
says.

Usov's three unique features would distinguish the strange star
from the more common neutron star, which is outwardly very
similar.

There are approximately 1,000 observed objects classed as neutron
stars in our galaxy, but, according to Usov, as many as one
percent of these may in fact be strange stars. So may be some of
the 20 or so enigmatic bodies which astronomers believe may be
black holes.

A champion of density and stability

The positive identification of these shadowy celestial denizens
would settle an important question for particle physics,
concerning the existence of matter made up of quarks -- the most
stable matter in the universe.

The existence of such matter was posited in 1984 by Prof. Edward
Witten of the Institute for Advanced Study in Princeton, and its
crucial component is the strange quark -- one of six types of
quarks, the tiniest building blocks of matter.

However, quarks do not normally exist as separate entities, and
it's only possible to get a brief glimpse of their existence in
high-energy particle accelerators.

Therefore, strange stars -- so called because they are believed to
consist almost entirely of strange quarks -- are scientists' only
chance to observe a sizable and stable "chunk" of quark matter.

"The discovery of a strange star would prove that quark matter can
indeed exist," Usov says.

Before Witten proposed his quark matter theory, it was assumed
that under the extremely high pressure existing in some
astronomical objects, matter made up of neutrons, which have no
electric charge, was the most stable of any matter type. This
stability led scientists to believe that stars made of neutrons,
which measure about 20 kilometers in diameter, were the final
stage in the evolution of a "massive" star (whose mass is at least
1.4 times greater than that of the sun). Neutron stars form when
massive stars collapse, and because they are exceedingly stable,
they are highly unlikely to collapse further.

However, the theoretical strange stars would represent an even
further stage in stellar evolution: according to Usov, when the
core of a neutron star is sufficiently dense, neutron matter can
be converted into quark matter.

Both neutron and strange stars are not only extremely stable but
also improbably dense: one cubic centimeter of strange quark
matter would weigh about 1 billion tons.

The three criteria

Although neutron and strange stars are similar in size and
density, Usov used theoretical calculations to search for unique
behaviors that would set a quark star apart from its neutron
"cousin."

These three unique behaviors are as follows:

First, the energy of X-rays emitted by a strange star is about 10
to 100 times greater than that of X-rays emitted by a neutron
star.

Secondly, the X-rays emitted by strange stars are fired in pulses,
each lasting around 1 millisecond.

Finally, the strange star, while comprising mostly quarks, also
contains a small quantity of electrons. As negatively-charged
electrons try to escape from the star, a very strong electric
field is created over its surface. This electric field causes
spontaneous creation of pairs consisting of electrons and their
positively charged counterparts, called positrons. The electrons
and positrons can annihilate each other when they meet, leading to
the release of high-energy gamma radiation. This so-called
annihilation gamma-ray emission can be detected by astronomers.

"If a compact object fits these three criteria, chances are high
that you've found a strange star," Usov concludes.

A prime candidate

According to Usov, one likely candidate for a strange star has
already been detected by astronomers. It's a mysterious X-ray
source located near the galactic center, referred to by scientists
as 1E1740.7-2942.

The powerful source is currently believed to be a black hole, but
Usov suggests it may be a strange star because it meets all three
of his criteria.

Usov is now working on a further refinement of the strange star's
profile in order to enable astronomers to identify other potential
candidates in the cosmos.

The Weizmann Institute of Science, in Rehovot, Israel, is one of
the world's foremost centers of scientific research and graduate
study. Its 2,500 scientists, students, technicians, and engineers
pursue basic research in the quest for knowledge and the
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